The present invention is related to a method and device for charging a battery and, more particularly, to a method and device for dynamically varying a current limit value for battery charging current.
First of all, it is pointed out that xe2x80x9cthe alternating current sourcexe2x80x9d herein is to be given a very broad sense and may for instance be an alternating current net, but also be formed by converting electric energy from a direct current supplying arrangement, such as a direct voltage net, into an alternating voltage, so that the xe2x80x9calternating current sourcexe2x80x9d is used as a summary for all types of configurations, which are adapted to supply an alternating current on the input of said rectifier.
A device of this kind may be used for charging all kinds of batteries, but to clearly illustrate the problems the invention tries to solve, such a device for charging a battery onboard a railway vehicle for electrified railway traffic will be explained hereinafter, but thus not at all for the purpose of limiting the invention. Such a battery onboard a railway vehicle is in most cases used especially to give an extra energy addition when starting the vehicle, but also to feed for example computers and lighting equipment in the vehicle. The alternating current consumer, which is fed in parallel with the battery, may be for instance fans, main compressors, lighting equipment etc in the vehicle or cars connected thereto. The electric energy is normally obtained by the vehicle taking, from a contact line 1 (see FIG. 1), electric energy, for instance in the form of an alternating voltage, which in Sweden normally is a single phase alternating voltage with a frequency of 16⅔ Hz, and that this via a rectifier not shown is converted into a direct voltage, of for instance about 750 V, which is then brought to a converter in the form of an inverter 2, where it is converted into an alternating voltage. The level of this alternating voltage is then changed by a transformer 3, from which the alternating voltage is fed further to consumers 4 of the kind mentioned. At the same time, a part of the alternating voltage is taken into a converter in the form of a rectifier 5, which rectifies the voltage and, at a higher voltage on its output 6 than the voltage of a battery 7, sends a charging current thereto for charging thereof. Said converters are preferably formed by current valves connected in series, each having at least one switch in the form of a semiconductor element and a rectifying diode connected in anti-parallel therewith controlled in a conventional manner according to a pulse width modulation pattern as in the embodiment shown in FIG. 2 or as non-controlled rectifiers, e.g. diode rectifiers, as in the embodiment according to FIG. 1.
From the contact line 1, the engine of the vehicle obtains electric energy also in parallel with the shown device, preferably by a not shown converter.
The charging current to the battery can be measured by schematically indicated members 8, whereas the voltage of the battery can be measured by members 9. The measurement values obtained in this manner are used for controlling the output voltage of the rectifier 5 and thereby the charging current to the battery 7. When the output voltage on the output 6 exceeds the voltage of the battery, the battery will be charged by the coil 10. The device also has an arrangement 11 adapted to control the voltage on the output of the rectifier based on information about a comparison of the measured charging current with a current limit value and decrease it if the measured current exceeds the current limit value for reduction of the charging current. This takes place at a device of the kind shown in FIG. 1 by controlling the inverter 2 so that a suitable voltage level is obtained on its output. However, this has the disadvantage that when the voltage to the rectifier 5 has to be decreased, the level of the voltage to the consumers 4 will then be reduced automatically.
FIG. 2 shows a variant of the device according to FIG. 1, at which said arrangement 11 controls the rectifier instead, so that the feed to the battery and that to the consumers practically become independent of each other.
In FIG. 3 it is shown somewhat more in detail how the arrangement 11 of a device of this kind may function. The value of the measured charging current arrives at 12 and its negative value is added in an addition member 13 to a current limit value arriving at 14. If the addition in 13, which in practice is a comparison of the charging current with a current limit value, which the charging current should not exceed to prevent triggering of overcurrent protection and stopping the operation of the entire device according to FIGS. 1 and 2 with interruption of the feed of electric energy to said consumers 4 as well as to the battery as a result, gives a negative value on the input 15 to a member 16, a reference voltage value for the voltage on the output 6 of the rectifier is output on the output of said member, and is then made lower than a predetermined voltage value input to the member 16 at 17. However, should the value of the charging current be lower than said current limit value, then the reference voltage input at 17 would also be output at 18. At 19, the battery voltage multiplied by xe2x88x921 is input to an addition member 20, which inputs the result of the addition to a member 21, which at a lower reference voltage than battery voltage sends signals to a control unit 22, which in the device according to FIG. 1 controls the inverter 2 as concerns voltage peak value and in the device according to FIG. 2 controls the rectifier 5 as concerns voltage peak value. If the battery voltage is higher than the reference voltage at 18, said voltage peak value and thereby the voltage out of the rectifier and thereby finally the charging current to the battery are reduced.
In FIG. 4 it is schematically shown what could happen if the level of the voltage into the rectifier suddenly decreases, for instance due to a large power output from a consumer when starting the same. It is schematically illustrated how the output voltage U6 varies over time, UB being the battery voltage. Furthermore, the charging current IB is drawn and said current limit value IB,lim is drawn with a broken line over time. It is apparent that when the output voltage from the rectifier decreases below the battery voltage, the charging current is lowered to 0, but when said output voltage then rises again above the battery voltage, the result due to the low internal resistance in the battery will be a very steep rise of the charging current, which often leads to a very large overshoot of the charging current. This implies that the charging current passes the current limit value and the arrangement 11 performs a control in the manner shown in FIG. 3 to lower the charging current below this value. However, the current overshoot is at times so large that over-current protection manages to trigger to prevent damages to transformers and diodes, so that the entire device is shut off with significant problems as a result. It is pointed out that voltage drops with corresponding problems as a result also can arise on the load side of the battery, where also large loads may occur.
The object of the present invention is to provide a device and a method of the initially defined kind, which find a remedy for the above mentioned problems with risk for triggering over-current protection upon suddenly rising output voltage from the rectifier. This object is obtained according to the invention by providing such a device with a unit adapted to form said current limit value by comparing a current value formed by the value of said measured charging current with a restriction on its positive rise speed with a predetermined maximum current limit value, and to determine the lowest of the so obtained current value and said maximum current limit value as said current limit value for the comparison of said means.
By dynamically varying the current limit value in this manner already when the charging current starts to rise, even if the current intensity then lies far below the value, which can really be accepted, the charging current will pass said current limit value if it rises faster than said positive rise speed and thereby the arrangement will control the voltage on the output of the rectifier to decrease for reduction of the charging current, so that the current intensity required for the over-current protection to be triggered is never reached. Accordingly, the invention is not based on that the regulation of the charging current is done faster, but it is started earlier instead, i.e. far below the maximally allowed level on the current. This way of regulation implies a very smooth way of restricting transients.
According to a preferred embodiment of the invention said unit comprises members adapted to add a maximum current value addition/unit of time to the value of the measured charging current and members adapted to compare a so formed current value with existing charging current and determine the lowest of them as a theoretical charging current value and send this to said adding members so as there to be considered as a new charging current value, and that the comparing member is adapted to send said theoretical charging current values to a member for comparing that with said maximum current limit value to determine the lowest of these as current limit value for the comparison of said means. Hereby it is ensured that the current limit value for the comparison of said means, which is the basis for the actual control of the output voltage from the rectifier, on one hand never gets higher than the value of the sum of the charging current and the maximum current value addition, which preferably at normal charging current implies that this sum becomes considerably lower than the maximally allowed current limit value, and on the other hand that the current limit value cannot rise too quickly. In practice, this implies that the current limit value will be exceeded by the charging current directly when it starts to rise faster than the positive rise speed allowed by said restriction, and this irrespective of the level of the charging current at that time, so that the regulation of the output voltage downwards can be started already at very low, but quickly rising charging currents.
According to another preferred embodiment of the invention the device is adapted to be a part of the electric system of a railway vehicle, and the alternating current source is adapted to obtain electric energy from a contact line for said railway vehicle. This contact line could be a direct current line as well as an alternating current line. The device is especially well suited for this application, as the problem which the invention tries to solve often appears in the electric system of such railway vehicles provided with devices for charging batteries.
The invention is also related to a method for charging a battery according to the appended method claims. The advantages with such a method are clearly apparent from the discussion above concerning the device according to the invention.
Further advantages with and advantageous features of the invention are apparent from the following description and the other dependent claims.